1. Field of the Invention
The invention relates to reference voltages, and more particularly to reference voltage generation circuits.
2. Description of the Related Art
A reference voltage generator provides a circuit with a reference voltage. An analog circuit needs a reference voltage as a reference for performing accurate operations. For example, both a least significant bit (LSB) of an analog to digital converter and an output voltage of a regulator are determined according to a reference voltage. Thus, a reference voltage generator must generate an accurate and reliable reference voltage to maintain circuit performance.
Most analog circuit components have electrical properties changing with temperature. To prevent the performance of a circuit from changing with temperature variations, even if the temperature changes, the level of the reference voltage generated by a reference voltage generator should not change with the temperature. Referring to FIG. 1A, a circuit diagram of a bandgap reference voltage generator 100 is shown. The bandgap reference voltage generator 100 generates a reference voltage Vref which has a zero temperature coefficient. In other words, the reference voltage Vref generated by the bandgap reference voltage generator 100 does not change with temperature. The bandgap reference voltage generator 100 comprises PMOS transistors 101, 102, and 103, diode-connected BJT transistors 130, 131, . . . , 13N, transistors 121, 122, 123, and 124, and an operational amplifier 150.
The operation of the bandgap reference voltage generator 100 is described as follows. The output voltage of the operational amplifier 150 is coupled to the gates of the PMOS transistors 101, 102, and 103, and the sources of the PMOS transistors 101, 102, and 103 are coupled to the voltage source Vcc. Because the voltage drop across the gates and the sources of the PMOS transistors 101, 102, and 103 are identical, the levels of the currents I1, I2, and I3 passing through the PMOS transistors 101, 102, and 103 are also identical (I1=I2=I3). Thus, the reference voltage Vref is derived as the following algorithm:
                                                                        V                ref                            =                            ⁢                                                                    I                    3                                    ×                                      R                    124                                                  =                                                                            I                      2                                        ×                                          R                      124                                                        =                                                            (                                                                        I                                                      2                            ⁢                                                                                                                  ⁢                            a                                                                          +                                                  I                                                      2                            ⁢                                                                                                                  ⁢                            b                                                                                              )                                        ×                                          R                      124                                                                                                                                              =                            ⁢                                                [                                                            (                                              Δ                        ⁢                                                                                                  ⁢                                                  V                          /                                                      R                            122                                                                                              )                                        +                                                                  V                        162                                            /                                              R                        123                                                                              ]                                ×                                  R                  124                                                                                        (        1        )            
wherein R124 is the resistance of the resistor 124, R122 is the resistance of the resistor 122, R123 is the resistance of the resistor 123, ΔV is the voltage drop across the resistor 122, and V162 is the voltage on the node 162.
Because a positive input terminal and a negative input terminal of the operational amplifier 150 are respectively coupled to the nodes 162 and 161, the voltage of the node 162 is identical to that of the node 161, and the reference voltage Vref is derived as the following equation:Vref =[(ΔV/R122)+V161/R123]×R124  (2)
wherein V161 is the voltage on the node 161. Because the voltage V161 on the node 161 is the voltage drop across the BJT transistor 130, the voltage drop V161 decreases with an increase of the temperature (referred to as a negative temperature coefficient). The ΔV is the voltage drop across the resistor 122. Because a plurality of BJT transistors 131, . . ., 13N are coupled between a terminal of the resistor 122 and the ground, the voltage drop ΔV therefore increases with an increase of the temperature (referred to as a positive temperature coefficient). Because the reference voltage Vref is a combination of the voltage drop V161 with a negative temperature coefficent and the voltage drop ΔV with a positive temperature coefficient, the reference voltage Vref does not change with temperature variations (referred to as a zero temperature coefficent).
Although the bandgap reference voltage generator 100 provides a reference voltage with a zero temperature coefficient, the bandgap reference voltage generator 100 has a deficiency. When the power of the bandgap voltage generator 100 is switched on, the voltage on the node 161 is at the voltage of the ground. The BJT transistor 130, however, is turned on when the voltage of the node 161 is higher than 0.7V. If the voltage of the node 161 is lower than 0.7V, the BJT transistor 130 is turned off, and the current I1 passing through the PMOS transistor 101 flows to the ground via the resistor 121 without passing through the BJT transistor 130. Because the BJT transistor 130 is not turned on, the voltage V161 on the node 161 does not have a negative temperature coefficient, and the reference voltage Vref generated according to the equation (2) therefore does not have a zero temperature coefficient. The bandgap reference voltage generator 100 therefore does not operate normally.
Referring to FIG. 1B, a circuit diagram of a starting circuit 170 of the bandgap reference voltage generator is shown. In one embodiment, the starting circuit 170 comprises PMOS transistors 171, 172, and 173, and an NMOS transistor 174. Because the BJT transistor 130 shown in FIG. 1A is not turned on when the power of the bandgap reference voltage generator 100 is switched on, the starting circuit 170 is added to the bandgap reference voltage generator 100 to pull up the voltage of the BJT transistor 130 after the power of the bandgap reference voltage generator 100 is switched on. However, even if the staring circuit 170 is added to the bandgap reference voltage generator 100, the BJT transistor 130 is not assured to always be turned on by the starting circuit 170, and the bandgap reference voltage generator 100 is not ensured of operating normally.
To avoid the deficiency of the conventional bandgap reference voltage generator 100 from occurring, a new-type bandgap reference voltage generator is therefore required.